Feeling the heat

In common with many people, as the weather’s turned colder, I’ve switched on the central heating. My boiler’s not even two years old yet, and is a pretty efficient Vaillant combi model, but like many such systems, it doesn’t have a built in timer. In fact, some combi installations don’t even have a room thermostat, just an on/off switch for the heating.

Some years ago, I replaced the existing thermostat in the hallway with a Drayton DigiStat 3. Even if you’ve never tinkered with a central heating system before, it’s actually a pretty straightforward job, because in many British heating systems, the thermostat on the wall does just one thing – it turns a circuit on and off; that may be a pump that drives water round the central heating system, or it may be the boiler itself, or something a bit fancier, but essentially, the wall thermostat is just a simple switch; you could, if you wanted, replace it with a light switch.

This means that if you want something a bit more sophisticated than a mechanical thermostat, it’s a simple job to replace your existing one. The Digistat 3 that I’m currently using is powered by two batteries, and as well as reacting to the temperature, includes a clock. It can be programmed with different temperatures for different times of the day – morning, daytime, evening and night – and days of the week.

That gives you a lot more control over your heating (and energy use), and combines both the function of a time switch and a thermostat. And it’s possible to add something like this to just about any central heating system because all you have to do is turn on and off a single switch – the boiler doesn’t care why it’s being told to turn the heating on or off, whether it’s time or temperature. The Digistat is just a relay, powered by batteries, and controlled by it’s clock and sensors.

Modern technology

While a programmable 7 day thermostat is perfect for a lot of people, it’s not always ideal for me. For example, sometimes I’ll be working in someone’s office, or spend the day out of the flat. On those days, it’s actually going to be a bit wasteful to have the heating running when I’m not here; it could be turned off, or the temperature turned down a few more degrees, saving more energy.

But, smart though the Digistat 3 may be, it has just four buttons and a small display. Making a change for a single day is a bit fiddly and so, like many people I suspect, I don’t bother. So, whether I’m working at home or not, the heating carries on with the same program.

One solution to that – and perhaps I’ve been talking myself into a new gadget here – is a thermostat that’s remotely controllable via the home network. By providing a web or app interface, it’s suddenly much easier to control than fiddling around with those buttons. It’s even possible to set things up to give me remote control, so I could turn on the heating when I’m just leaving the office, for instance, or stop it coming on if I’m going to be working late.

I’ve been thinking about this because it looks like the faithful old Digistat is on its last legs; last week the relay appeared to be stuck in the ‘off’ position, and this week it died completely for a few hours, prompting me to search for a WiFi thermostat.

WiFi thermostats?

Yes, some of you are probably wondering if the world’s gone mad. But a quick search online revealed that there are several out there, though many are made for the US market, and while they may be available in the UK, or by mail order, that’s not ideal.

One of the main reasons for that is that in the US, systems that incorporate both cooling and heating are much more common; wireless thermostats tend to be a fairly high end product, and so they support all the options. And from a bit of cursory reading, it seems that as a result, the connections to a thermostat in the US tend to be a little more complicated. Of course, you can buy a fancy thermostat that supports aircon as well as heating, and tell it you just have a simple on-off control, but that does seem to be a bit of overkill.

And, you’ll still run into wiring issues; a wifi thermostat isn’t going to run on batteries. The US models that I’ve looked at expect there to be a 24 volt supply available through the thermostat wiring, and that’s not going to be much use to you in the UK. (Though oddly, there are terminals in my boiler that the manual indicates would provide the required power; they’re marked “not to be used in the UK”.)

Wiring up

Fortunately, there are WiFi thermostats available for the UK market too, including several models from Heatmiser, which I’ve been looking at; there’s even an app for remote control.  I was sorely tempted by one, with WiFi, 7 day programming and a touch screen. But there’s one sticking point – the need for a 230 volt supply.

Heatmiser thermostat
Heatmiser's WiFi thermostat

To make the Heatmiser WiFi stat work, you’ll need at least three wires, plus earth, coming into the back of your existing thermostat. Then, when it’s connected up, two of those will supply live and neutral to the thermostat itself, and the third will feed back the live supply to the boiler when heating is required.

If you have three core plus earth cable, then you’re all set; you might have to adjust some connections at the boiler end, but at least you won’t have to run a new cable through.

Unfortunately, I don’t. Since my previous thermostats have only ever acted as a dumb switch, the wiring into my thermostat is standard mains cable – two cores, plus earth. I suspect that an awful lot of people are in that position.

It would be possible to replace the wire; it’s not buried behind plasterwork in my flat, but it does run along skirting boards, behind fitted kitchen cupboards, and through the back of a fireplace. Ripping out the existing one could be done, but it’s the sort of DIY that is just a hassle, even if it doesn’t involve the replastering and repainting that many people would find necessary.

Fortunately, it turns out that Heatmiser is working on a new version, which uses WiFi to allow control of the thermosat, but a separate wireless circuit for the actual switching. That means that the bit that actually turns the boiler on and off can be sited right next to it, and the touch screen panel WiFi unit replaces the existing wall thermostat.

Since it’s controlling the switch by wireless, all it needs is power, live and neutral. Which means that the existing two wire connection can be reused to do that, and I won’t have to spend hours trying to coax a new cable out from behind the kitchen cabinets.

They’re going to send me a unit to test, and I’ll write more when I’ve got it.

9 Replies to “Feeling the heat”

  1. I’ve been looking in to the exact same thing and came to the conclusion it was a lot of expense for something that still didn’t really do what I wanted. I need to replace my existing wireless thermostat/timer but I think I might just get a better version of the same thing. Let us know how you get on!

  2. Actually, your old mechanical thermostat probably wasn’t wired properly in the first place !

    For as long as I’ve had any interest, and more, most mechanical room thermostats have had a compensating heater which means they need a neutral connection. Without it they tend to have possibly several degrees of hysteresis between switch off and switch on temperatures which causes the room to heat up and cool down in an uncomfortable manner.
    It could be that the person who wired it up used the earth for this. It’s wrong, but I’ve seen worse.

    If you really want to go overboard, you can get systems that will control every room individually, and connect to your network for setup and monitoring. Do be sitting down before you look at the prices though.

  3. @Simon H: actually a 3-wire’s wiring is not quite how you say: it has two phase conductors and one neutral. It takes the permanant phase and uses it to drive the heater and to switch to the switched phase wire. Then common via the neutral. The boiler doesn’t look at the permanent phase, just the switched phase, this is how you wire a 2-wire thermostat. All the boiler sees is an on/off switch. When the thermostat is absent the connectors are wired with a permanent link.

    And, I agree, running neutral via the earth conductor is really naughty/stupid. There is no requirement for E & N to be at the same potential; usually are close. It is also very bad practice because there is then no earth conductor the unit, and if the house wiring isn’t 17th edition you have no leakage protection at all.

    Most boilers just need to know if it is ok to be on or not: timer+temperature. This is traditionally achieved by having two switches in series: when time and temp are closed then current flows.

    It is annoying that you need WiFi to do the control, because the bandwidth you require is so tiny. There are low bandwidth protocols, such as ANT+, but only works over 3m. That will work off a single CR2032 for several years.

    In a real house the enduser control panel is fixed, and is usually in the same place as the old mechanical thermostat, so I don’t see this as a problem. Kinda silly that your clever controller just uses the wire to get juice and then uses wireless to get the signal, when the original copper pair can do the job nicely.

    No standards on the horizon for clever over-wire protocols and hardware? Then you can have a slave unit by the boiler and the panel elsewhere. Some panels/slaves communicate over wire, and others via some wireless.

    1. Effectively, the unit I should be receiving, is a panel / slave, with an RF link, allowing the reuse of the existing wire just to power the panel. I don’t know what protocol is used to signal the slave, but I don’t think it’s wifi too – that’s just used to enable the IP control of the panel.

      I suppose the issue with implementing something clever over the mains wiring is that there are doubtless regulatory issues and a lot of work in coming up with a new protocol, not to mention the fact that in an increasing number of homes, there may be other technologies signalling over the mains, like HomePlug, and you’d have to ensure you didn’t interfere with that too.

      Embedding HomePlug in the thermostat, and in a slave receiver would probably put the cost up quite a bit, and complicate things more; meanwhile, there are lots of modules easily available to perform things like switching functions via RF in the unlicensed bands

  4. Nigel there isn’t a problem with regulation in that the twin/three and earth cable is point to point between the user panel and the boiler. Any weird protocol can be used because it the implementer thought it might be a problem he can put filters in his boiler board electronics.

    And as you say, you can use WiFi to talk to the system too, as an addon goodie. This does make a lot of sense, in that via your firewall you can serve the controls so you can remotely switch your heating from your smartphone.

    Using RF as the primary control channel to your boiler does sound like it would make it less reliable. A client of mine had a Buderus wireless controller: he took it out and replaced it with a wired one. He couldn’t use a standard one because this German boiler is so “clever”.

    But good luck.

  5. Hi all
    Is there any news on the new thermostat cos I’ve been hunting for a wifi unit that will work on the 2 wire system
    Cheers Dave

    1. Slight delay in getting it to me thanks to Xmas, but should have it soon, and there’ll be a review on RegHardware shortly after

  6. I look forward to a review.

    We recently moved into a new (to me, not new build) home and are already missing the Honeywell 7 day wireless thermostat we used to have. The ability to move the stat around the house was very useful.

    New house has a traditional mechanical stat with 4 wires, I think I should be able to replace it with the Heatmiser unit for more effective heating control.

    Do let us know how it goes.

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